Role of Solvent Size in Ordered Ionic Structure Formation in Concentrated Electrolytes for Lithium-Ion Batteries

The structural and electrochemical properties of lithium (Li) ion complexes in concentrated electrolytes based on acetonitrile (AN) and tris(2,2,2-trifluoroethyl)phosphate (TFEP) as solvents and LiTFSA [TFSA: bis(trifluoromethanesulfonyl)amide] as a Li salt were investigated by employing electrochemical measurements, vibrational spectroscopy, and high-energy X-ray total scattering (HEXTS) with all-atom molecular dynamics (MD) simulations. Via electrochemical measurements, reversible Li-ion insertion/deinsertion into/from the graphite electrode was observed in concentrated LiTFSA/AN solutions but not in concentrated LiTFSA/TFEP solutions. The experimental radial distribution functions [G(exP)(r)] derived from HEXTS were successfully represented by the corresponding MD-derived values [G(MD)(r)] for both AN- and TFEP-based electrolyte systems. We found that (1) in the dilute system, Li ions were solvated with only solvent molecules in AN-based solutions to form a completely dissociated [Li(AN)(4)](+) complex, while contact ion pairs exhibiting Li+center dot center dot center dot TFSA(-) interactions were formed in the TFEP-based solutions. (2) In the concentrated system, a specific Li+center dot center dot center dot Li+ correlation was observed for shorter r values (similar to 3 angstrom) in the AN-based solutions, suggesting ordered ionic structure formation based on multinuclear Li-ion complexes. However, no ordered ionic structure formation was found in the TFEP-based solutions. We discussed the relation between the ordered ionic structure and graphite electrode reaction at the molecular level, particularly focusing on the solvent size; that is, the smaller AN more easily forms a compact solution structure (ordered structure) in the concentrated solutions, while bulky TFEP causes steric repulsion among the coordinated species (TFEP and TFSA) in the Li-ion complexes, preventing such ordered formation.